Sanjoy K. Gupta

Temporal changes in PO and MBP gene expression after crush-injury of the adult peripheral nerve

The crush-injured sciatic nerve provides a model to study Schwann cell regulation of myelin gene expression during the process of demyelination and remyelination. In order to investigate the possible transcriptional regulation of myelin gene expression, the quantity, quality and translational efficiency of PO (the major myelin glycoprotein) and MBP (the myelin basic proteins) coding messages were investigated as a function of time following crush-injury of the adult rat sciatic nerve. Northern blot analysis indicated that the size of the PO and MBP transcripts remain unchanged in the distal segments of crushed sciatic nerves at 1, 2, 4, 7, 10, 14 and 21 days after crush-injury. Dot-blot analysis showed a sharp drop in levels of PO and MBP coding transcripts 1 day after crush-injury with the lowest steady-state levels at 4-7 days. Message levels were found to increase after 7 days, the highest increase in levels of message was found to be between 10 and 14 days. The highest steady-state level of both transcripts was observed at 21 days. In vitro translation and immunoprecipitation of PO-translated products from various stages of crush-injury also indicated this trend. The pattern of gene expression of PO- and MBP-coding transcripts parallel each other and follow the pattern of demyelination and remyelination. The results are also consistent with our previous interpretation which suggests that PO and MBP gene expression is regulated at the level of transcription and that these two genes might be coordinately expressed. Western blot analysis of PO protein from these stages revealed a similar decrease and then increase in the levels of the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Myelin-associated glycoprotein gene expression in the presence and absence of Schwann cell-axonal contact

The distal segments of the crush-injured and permanently transected sciatic nerve provide models to study Schwann cell activity in the presence and absence of Schwann cell-axonal contact, respectively. We examined the quantity and quality of transcript coding for the myelin-associated glycoprotein (MAG) over a 3-week period following crush injury and at 35 days after transection to investigate possible regulation of this gene during nerve injury and subsequent repair. Northern blot and slot blot analysis indicated a sharp decrease in levels of MAG mRNA 2 days after crush injury which was followed by a progressive increase in levels of message between 7 and 21 days after injury. Western blot analysis showed that levels of MAG protein decreased substantially 7 days after crush injury, which returned to 70% of the adult value by 21 days after injury. MAG mRNA and protein were undetectable by Northern and Western analysis, respectively, in the distal segment of the sciatic nerve 35 days after permanent transection. This infers distinct down-regulation of MAG gene expression after permanent transection of a peripheral nerve. These comparative studies of MAG transcripts and encoded protein may indicate regulation of MAG gene expression at the level of transcription, and possibly at the level of post-transcription in these experimental models of peripheral neuropathies.

Genotype/phenotype correlations in aniridia

PURPOSE To detect and characterize mutations in cases of familial and sporadic aniridia in Maritime Canada, and to look for indications of genotype/phenotype correlation within the cohort. METHODS Twelve consecutive and unrelated patients (probands) who had total or nearly complete absence of irides, and four affected relatives, were recruited from Maritime Canada. Clinical data were obtained by chart review and electroretinogram testing. Mutations in the PAX6 gene were detected by single-strand conformation polymorphism and characterized by sequence analysis. RESULTS Eleven different PAX6 mutations, 10 of which are novel, were found. The four patients with congenital cataracts all had mutations in the C-terminal proline-serine-threonine (PST)-rich domain of the PAX6 protein. Electroretinograms of nine of 11 patients displayed depressed scotopic maximum response b-wave amplitudes. The greatest decrease in b-wave amplitudes was seen in patients in whom the paired domain was disrupted by mutation. CONCLUSION Some aspects of the phenotype of aniridia appear to correlate with the predicted effect of point mutations on the paired and PST domains of the PAX6 protein.

Leber's congenital amaurosis with anterior keratoconus in Pakistani families is caused by the Trp278X mutation in the AIPL1 gene on 17p

BACKGROUND Lebers congenital amaurosis (LCA) represents the earliest and severest form of retinal dystrophy leading to congenital blindness. A total of 20% of children attending blind schools have this disease. LCA has a multigenic basis and is proving central to our understanding of the development of the retina. We describe the clinical and molecular genetic features of four inbred pedigrees from neighbouring remote villages in northern Pakistan, in which some of the affected members have concurrent keratoconus. METHODS History-taking and physical and eye examinations were performed in the field. Venipuncture, DNA extraction, studies of linkage to known LCA genes, automated sequencing and polymorphism analyses for haplotype assessments were done. RESULTS We examined 12 affected and 15 unaffected family members. By history, there were an additional nine blind people in the four pedigrees. In each pedigree a consanguineous marriage was evident. We found a homozygous nonsense mutation in the AIPL1 gene, which replaces a tryptophan with a stop codon (Trp278X). The phenotype is severe and variable, despite the common molecular genetic etiology in each family. Affected patients had hand motion to no light perception vision and fundus findings ranging from maculopathy to diffuse pigmentary retinopathy. Three affected members had definite keratoconus, and two were suspects based on mild cone formation in the cornea of at least one eye. INTERPRETATION We have identified four Pakistani families with a severe form of LCA that is associated with severe keratoconus in some affected members. The molecular etiology in all four families is a homozygous nonsense mutation, Trp278X, in the photoreceptor-pineal gene AIPL1. To our knowledge, this is one of the first phenotype-genotype correlations of AIPL1-associated LCA.

Lattice corneal dystrophy type 1 in a Canadian kindred is associated with the Arg124 → Cys mutation in the kerato-epithelin gene*

PURPOSE To identify the mutation responsible for lattice corneal dystrophy type 1 in an extended Canadian kindred. METHODS A search for a mutation in the candidate gene, kerato-epithelin, was carried out by single-strand conformation polymorphism and sequencing analyses. RESULTS AC --> T mutation at position 417 was detected in exon 4 of the kerato-epithelin gene, which is expected to cause an Arg124 --> Cys change. This is the same nucleotide change described previously in two Swiss families with lattice corneal dystrophy type 1. CONCLUSION Although the possibility that the three families (two previously described Swiss families and this Canadian kindred) are related has not been excluded, it appears that the unique phenotype of lattice corneal dystrophy type 1 is caused by this particular amino acid change.

A Frame shift mutation in a tissue-specific alternatively spliced exon of collagen 2A1 in Wagner’s vitreoretinal degeneration

PURPOSE To describe the genetic basis of an autosomal dominant vitreoretinopathy in a large French-Canadian kindred. METHODS A clinical cohort study followed by laboratory-based genetic and molecular analysis. Thirty-two affected and 22 unaffected members of the kindred were examined. Candidate genes/regions for Wagners disease and Stickler syndrome were tested for genetic linkage. Mutation analysis was carried out with direct PCR-based sequencing. RESULTS Funduscopic examinations of 32 affected patients revealed optically clear vitreous, vitreous veils, and radial perivascular pigmentation. Spondyloarthropathies or craniofacial abnormalities were notably absent. There was a 53% rate of retinal detachments that required surgical intervention. Genetic linkage was obtained to COL2A1, the candidate gene for Sticklers type I. A frame shift mutation in exon 2, leading to early truncation of the protein (Cys57Stop), was detected. CONCLUSIONS Wagners disease in this large kindred has had devastating visual consequences. In affected individuals, we found a novel COL2A1 frame shift mutation in exon 2. The mutation arises in an exon that is selectively present in vitreous collagen mRNAs, but absent in cartilage mRNAs through tissue-specific alternative splicing. Tissue-specific alternative splicing of COL2A1 mRNAs thus provides an elegant biochemical mechanism for a clinical phenotype of Wagners disease in this kindred.

Levels of Proteolipid Protein mRNAs in Peripheral Nerve Are Not Under Stringent Axonal Control

Abstract: The proteolipid protein (PLP) is the major protein in the myelin sheath of the CNS. It was recently reported that PLP coding transcripts are also found in the PNS, although the protein was not detectable in peripheral nerve myelin. In the present investigation, levels of mRNA for PLP in sciatic nerve were studied during development and following transection and crush injury. Results were compared to those for P0, the major PNS myelin protein, and the myelin‐associated glycoprotein (MAG). PLP transcript levels were very low at 21 days in sciatic nerve and remained unchanged in the adult sciatic nerve. This contrasts markedly with P0 and MAG mRNAs, which are expressed at high levels during development and decrease in content significantly by adulthood. The level of PLP messages was reduced ∼40% in the quiescent Schwann cells in the distal segment of the sciatic nerve at 21 days after permanent transection, yet P0 mRNA levels were very low, and MAG mRNAs were undetectable in this tissue. The distal segment of the crush‐injured sciatic nerve is characterized by transient demyelination followed by rapid myelination. PLP mRNA levels remained comparatively unaffected in the 3‐week period following crush injury. RNase protection experiments using two antisense riboprobes confirmed that levels of PLP‐derived protected fragments, corresponding to PLP and DM‐20 messages, remained unchanged in the developing and adult sciatic nerve. These results indicate that myelin‐specific P0 and MAG genes are tightly controlled at the level of transcription through Schwann cell‐axonal interactions, whereas PLP transcription in the peripheral nerve remains nearly dissociated from axonal influences.

A new clinical perspective of corneal dystrophies through molecular genetics.

In the past 2 years, significant advances have been made in the genetics of corneal dystrophies. Genetic heterogeneity (one disease condition caused by single mutations in any one of multiple genes) and phenotypic diversity (many disease conditions caused by mutations in a single gene) are common emerging themes. Genetic heterogeneity in Meesmann corneal dystrophy was established with the identification of two causative genes, keratins 3 and 12, that encode cytoskeletal proteins. Conversely, mutations in a single gene, keratoepithelin, were found to cause several distinct corneal dystrophies affecting the Bowman layer and the stroma. We present a novel preliminary classification of corneal dystrophies based on molecular etiology. This classification may be useful in understanding the pathogenesis of corneal dystrophies and in developing new strategies to treat these dystrophies.